The invention relates to a method for the thermal treatment of moist hydrates.
A method of this kind is for example the so-called calcining of moist gypsum. Moist gypsum, i.e. calcium sulphate dehydrate, which also includes moist gypsum from flue-gas desulfurization plants can be surface-dried at temperatures up to 90.degree. C. without removing the water of crystalisation. At temperatures up to about 145.degree. C. the withdrawal of the water of crystalisation takes place, i.e. the dewatering to the hemihydrate. The alpha or beta modification of the hemihydrate thereby arises, depending on the water vapour partial pressure. Above this temperature there arises, depending on the temperature level, through full yielding up of the water of crystalisation, the anhydrate modifications, starting with the so-called A III gypsum, a very reactive component with a short setting time, followed by sluggishly reacting hardly soluble A II s and the inert A II u. These distinctions in the reactive behaviour are tributed to the surface characteristic which is changed by the thermal treatment.
In order to obtain calcined gypsum with a constant and/or pure phase composition it is thus important to precisely control the temperature profile during manufacture. No overheating and no inhomogenities are permitted during the thermal treatment which with individual particles of the material could lead to undesired phase conversions. This applies to all thermal steps, i.e. also to drying.
There are presently no methods with which the above named maxima can really be satisfied. No methods have separate drying and calcining stages. For the drying stage flow tubes or combinations of heated mixers and flow tubes are used which are operated with flue gases or with indirectly heated gases of over 200.degree. C. For the contact surface heating which is used the customary heat transfer medium for example oil or steam are used from about ca. 150.degree. C. upwards. It is not possible to prevent the material being at least partially overheated through too long contact with hot gas flows or heated surfaces and the undesired phases, for example A III, A II, already form here during the drying.
For the calcining so-called cookers, drums, rotary tubes with heated surfaces ("steam tube") and also diverse furnace types are used. Directly fired drums and furnaces originate from the conventional gypsum burning technology and are only suitable for lump material.
Cookers and steam tubes are charged with a fluidizable gypsum powder (granulate) such as arises from flue gas desulfurising plants.
Characteristic for the said apparatus is either the direct firing with hot gases (flue gases or air) as a rule over 400.degree. C. and/or the incomplete fluidization in the area of heated surfaces.
In this way one arrives in just the same way as the dryers to local overheating and uncontrolled formation of multiphase gypsum. Moreover flue gases are not inert and no controlled atmosphere (for example H.sub.2 O partial pressure can be set). These disadvantages of the calciners are adequately known and thus the subject of improvement proposals. In practice one compensates for these process disadvantages in that one subsequently influences the product characteristics by the differential addition of additives, so-called set-up agents, or by additional method steps (grinding, cooling).
The known proposals are aimed at removing the detrimental excess temperature from flue gases by prior indirect heat exchange, at using fluidized bed apparatus to intensify and homogenize the heat transfer and also not allowing flue gases to come into contact with the product.
Thus a fluidized bed apparatus operated with hot air is described in FR-PS 1 338 126 which can however only incompletely solve with a steering mechanism the fluidization problems in the infeed zone, whereby again the danger of local overheating exists, and indeed in particular at the flue gas streams leading through the layer. The process of DE-PS 37 21 421, a fluidized bed with an inert granulate filling, such as for example sand and gas side circulation circuit, avoids these disadvantages. DE-PS 26 22 994 describes a fluidized bed apparatus which attempts to improve the inhomogeneous product quality by smoothing the dwelltime with the aid of chambers. This apparatus has the disadvantage that in order to achieve an adequate drying power and a degree of efficiency it is necessary to operate with high hot gas temperatures--with the known danger of overheating.
In GB 2 027 859 A an apparatus similar to a cooker is offered which cools down the flue through heat exchange surfaces in the product layer before the flue gases are used for (partial) fluidization. The DE-OS 37 38 301 picks up this principle of indirect heat exchange. Moreover, an indirectly heated hot gas is used for fluidization, no flue gas, and an attempt is made to homogenize the dwelltime to differentially fluidized chambers. In these two processes the danger of overheating at the heat transferring walls also arises, in particular with partial or differential fluidization. Moreover all previously described processes --with the exception of the granulate fluidized bed (DE-PS 37 21 421)--require a conventional dried and thus, as explained above, already damaged dihydrate powder.